As the size of semiconductor devices decreases, it becomes much more difficult to accurately measure the electrical characteristics of the devices. This is particularly true for very large scale integrated circuits fabricated on semiconductor wafers with very small spacing between components, especially those fabricated with low resistance metals and other types of electrically conducting materials. Currently, there are several methods and associated equipment for measuring electrical characteristics of complex transistor-size integrated circuits. These methods include scanning capacitance microscopy and the use of scanning tunneling microscopes (STM), as well as atomic force microscopes (AFM).
One method common to the art involves using a scanning probe connected to a voltmeter to measure the voltage drop across a sample of electrically conductive material attached to a current source. Accurate measurements are sometimes difficult to obtain with this method because the electric potential of the contact between the probe and the sample may vary with position and be large enough to affect the measurement.
In order to overcome problems associated with contact potential, a second type of measurement uses a voltmeter permanently affixed to opposite ends of the sample. An electron microscope may then be used as a current source and used to supply current to selected portions of the sample. While this method successfully overcomes the problems associated with the electric contact potential, it is also sometimes difficult to achieve accurate results. In order to use an electron microscope, the sample must be measured in a vacuum. Also, the available beam current is small in electron microscopes and this limits accuracy.
Many of the methods and equipment used for measuring electrical characteristics of integrated circuits at a microscopic level lack sufficient spatial resolution. It is difficult to conduct reliable measurements on microscopic devices or samples.